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Trouble Shooter

Vehicle computer circuits are sensitive to power- or ground-side voltage drops. Without reliable power supply voltage and ground, control units may make unpredictable decisions.,

Miss Replacement

I am working on a 1995 Pontiac Sunfire with the 2.2L engine. The vehicle has 120,000 miles on it and the No. 1 and 4 cylinders are misfiring. When I unplug the alternator, the engine runs good with no misfire. I have replaced the alternator twice, using parts from two different suppliers. Other parts that have already been replaced include one GM fuel pump and another from an aftermarket manufacturer, two GM ECMs, the crank and cam sensors, the ignition module, the ignition coils, the spark plugs and every other engine management-related sensor and switch, times two. I’ve also checked the catalytic converter and timing chain. I’ve removed the entire wiring harness to check it for damage. Someone suggested the processor in the instrument cluster might be to blame, so I changed that, too. The engine still has a bad miss that goes away when I unplug the alternator. It’s a nice car and I’d like to get it running right again. Any help would be greatly appreciated.

Terry Williams

West Jefferson, OH

This may sound like a wild guess, Terry, but I don’t think this vehicle is going to be repaired by replacing another part. Pretty much everything has already been replaced, which is probably why you’re reaching out.

I suggest you begin by checking (or rechecking) for abnormally high voltage drops on any of the circuits related to the engine management system, particularly the grounds. It sounds like the PCM is making incorrect fuel trim decisions because it’s receiving an unstable input voltage and/or ground from the charging system. When you disconnect the alternator, the system relies on battery power only. Assuming the supply and ground circuits between the battery and PCM are okay, the PCM starts making better decisions and the misfire goes away.

Begin by checking the charging system ground. With the engine running and the alternator operating, measure the voltage drop from the alternator case to the negative post on the battery. Set your DVOM on the DC volt scale, using autoranging if your meter has that capability. If it doesn’t, use the smallest voltage scale possible, without overranging the meter. The closer you match the DVOM scale to the actual voltage drop, the more accurate your reading will be.

Find a clean spot on the alternator case for one of your meter leads, then securely attach the second lead to the battery negative terminal. The voltage drop on the alternator ground circuit should be no more than .25 volt. Swap the leads if you get a negative reading. On computer circuits, drops of less than .1 volt are expected and preferred, due to the fact that they usually handle relatively low current flow. On a high-current-flow circuit like the charging system, slightly higher voltage drops are expected and permissible.

If the alternator ground looks questionable, make sure it’s making a clean electrical connection to the engine, then repeat the voltage drop test. If the result is still out of limits, move on to the main engine and transmission grounds. Remember, the alternator isn’t physically connected and grounded to the battery, and must rely on the engine and transmission grounds to complete its path to ground. Even a seemingly small voltage drop can cause this type of problem because the PCM must have a reliable voltage supply and ground to make the proper engine management decisions. If you throw a rogue voltage drop into the mix, all bets are off.

If the engine misfire persists after the alternator, engine and transmission grounds have been repaired and verified, move on to the PCM power supplies and grounds. The PCM on your customer’s Sunfire is powered by two fused ignition feeds and one fused battery feed. Three of its terminals (J1-24, J2-25 and J1-23) are connected to ground at the same grounding point. Refer to a vehicle wiring diagram for its exact location. The ground wires for two (J1-24 and J2-25) are spliced together before they reach ground, so a poor splice also could be causing problems.

Repeat your voltage drop circuit testing, this time on each of the PCM ground and power supply terminals. Remember, the voltage drop on these circuits must be .1 volt or less and the tests must be conducted with the engine running and the circuit “live.” Terminal 5 of the DLC connector also shares its ground source with the PCM ground terminals, and can be used for a quickie PCM ground circuit voltage drop test if the PCM terminals are not easily accessible. If the quickie test at the DLC reveals a problem, dig out the PCM for further testing. It doesn’t take much of a voltage drop to foul things up, so make sure all of these circuits have a clear and unobstructed electrical path to the alternator and battery before you replace any other parts. Good luck.